1. Field of the Invention
The present invention relates to instrumentation in an environment requiring accurate and continuous extraction of test data during operation of the instrumentation device and of the system in which the device is operating. More particularly, the present invention provides a fully testable analog signal conditioner for thermocouple signals from which process parameters are derived from such industrial processes as the generation of electric power by nuclear energy.
2. Description of the Prior Art
Analog trip devices are relatively well known in the instrumentation art. Modularization of such devices is also known. For example, in the nuclear power plant environment, Rosemont Corporation manufacturers a line of analog trip modules designated analog control units.
Bailey Meter Company has previously built signal conditioning printed circuit boards using a bridge type circuit for all signal conditioning and offset adjustments. All adjustments in the bridge type of circuit are interactive with each other. The Bailey signaling condition circuitry is not testable while installed and during normal circuit operation.
One analog trip device that is capable of being tested at any time during normal system operation is described in U.S. patent application Ser. No. 402,371, filed July 27, 1982, now abandoned, entitled APPARATUS AND METHOD FOR GENERATING TRIP SIGNALS. The described analog trip module generally operates in a signal range of 1-5 volts. Such level, while effective for trip module operation, is excessive for signal conditioning applications.
Thermocouples provide a signal in the range of from 2-120 mV (millivolts). Existing self-testing techniques are not applicable to the sensitive analog signal conditioning equipment needed with the low level signals provided by thermocouples.
The modular approach to instrumentation signal conditioning equipment necessitates the use of a common circuit for different thermocouple signal levels. The prior art, where self-testing has been available, does not offer a means for testing signal conditioning equipment operating at various signal sensitivity levels. Indeed, most such signal conditioners are not even adjustable to operate at different levels of sensitivity. Furthermore, the bridge-type configuration of prior art signal conditioners does not allow for simple substitution of thermocouple types. Changing a thermocouple type to accommodate different measurement parameters at various process locations, unbalances the bridge. In practice, a new bridge circuit must be supplied for each different thermocouple type to be used with the signal conditioner and for each sensitivity setting.
Finally, the bridge approach of the prior art signal conditioners does not allow for independent adjustment of other parameters, such as signal offset. The interaction between the various adjustments in prior art signal conditioners, due to the limitations of bridge-type level detection, necessitates the commitment of considerable time to signal conditioner alignment and calibration by trained maintenance personnel. Such adjustments, in addition to being difficult to make, tend to lower the reliability of the signal conditioner. This problem is particularly pronounced in equipment operating at low signal levels, such as those at which the signal conditioner operates, some of which are on the order of a few millivolts.
There has heretofore been no signal conditioning equipment for process instrumentation applications that is testable in situ during system operation, modular so that it may be readily used with different thermocouples and at different, selectable gain levels, and that provides a plurality of selectable, independent signal parameters for shaping circuit response characteristics.